Preparation of an article in glass or in glass-ceramic with improved coating and said article

ABSTRACT

The object of the present invention is a method for preparing an article, the structure of which comprises a substrate in a glass or in a glass-ceramic and an original coating fixed to at least one portion of the surface of said substrate, in a characteristic way, said coating, most particularly proposed with reference to the technical problem of VOC emission when it is brought to high temperature, is generated from at least one loaded polysilsesquioxane resin; said at least one polysilsesquioxane resin not containing any aryl radical in its structure. The object of the present invention is also the article which may be obtained after carrying said method.

The present invention is located in the context of glass orglass-ceramic articles, with a loaded coating on at least one portion oftheir surface. The involved coating is optimized with reference tospecifications which include many stipulations, notably those specifiedhereafter and more particularly the following stipulation: low emissionof volatile organic compounds (VOCs) when said coating is brought tohigh temperatures of the order of 100-500° C. The present invention moreparticularly relates to a method for preparing (obtaining) articles ofthe type specified above as well as to the articles which may beprepared (obtained) by said method.

Such articles may notably consist in cooking hobs, more or lesstransparent per se and made opaque by a coating covering at least theirface (intended to be positioned) facing the heating elements, so as tohide said heating elements as well as the adhesive product (of thesilicone adhesive type) ensuring the seal, used for placing said hobs.The field of application of the invention is however not limited to thiscontext of cooking hobs.

According to the prior art, cooking hobs mainly in glass-ceramic and inglass, notably tempered glass, have already been described, with acolored coating on their “lower” face (or reverse face or undersideface), i.e. the one intended to be positioned facing the heatingelements.

In patent application EP-A-0 277 075, the presence of decorative ormarking elements is very generally provided on the reverse face of aglass-ceramic plate, intended for cooking by induction heating. Nodetails on the involved decorative or marking elements are provided.

In patent application EP-A-1 267 593, transparent glass-ceramic platesare described with colored coatings underneath. Said plates are suitablefor induction heating, or even combined heating: heating by inductionand radiant heating. The involved colored coatings are enamels withmetal pigments, themselves optionally covered with an organic coating(such as polyimide, polyamide, fluorinated, silicone resin) on the “hot”surfaces.

In patent application EP-A-0 861 014, a paint layer resistant to hightemperature, preferably to temperatures above 650° C., is provided onthe whole of the face opposite to the one intended to receive thecooking utensils. Glass-ceramic heating hobs are involved but no detailson the exact nature of the mentioned paints are provided.

The WO-A-2005/092810 application describes different kinds ofglass-ceramic or glass plates with a colored coating consisting:

essentially in a mixture of inorganic fillers and of a crosslinkablepolysiloxane resin,essentially in a mixture of inorganic fillers and a crosslinkedcrosslinkable polysiloxane resin, oressentially in a mixture of inorganic fillers and of a pyrolyzedcrosslinked crosslinkable polysiloxane resin (=a porous mineral matrixbased on silica).

The inorganic fillers exert a coloration and reinforcement effect withinthe resin.

The WO-A-2005/092811 application describes the reinforcement of glass orglass-ceramic plates with such coatings.

Polysiloxane resins, the use of which is recommended in both of theselast documents of the prior art, advantageously contain phenyl groups,optionally phenyl and alkyl groups, insofar that said phenyl groups haveproved to be necessary for giving the coatings both good stability intemperature and good mechanical properties. It is generally known to oneskilled in the art, that high temperature resistant silicones arephenylsiloxanes and that the alkyl groups have a strong influence onmechanical flexibility.

Other types of coating have been proposed for glass or glass-ceramicsubstrates, notably according to the teachings of U.S. Pat. No.6,863,923 and of application US-A-2009/0233082. These coatings are basedon a binder of the sol-gel type, loaded with pigments. The handling ofthis type of binder imposes constraints insofar that the latterconstantly changes over time, because of hydrolysis and condensationreactions which occur inside it, even at room temperature. The lifetime(“pot life”) of this type of binder is therefore very limited.

Moreover in parallel with the chemistry of polysiloxanes (polymers with(R₂SiO) units, each silicon atom being bound to two oxygen atoms), thereexists a chemistry of polysilsesquioxanes (polymers with (RSiO_(1.5))units, each silicon atom being bound to 3 oxygen atoms). The propertiesof said polysilsesquioxanes are of course dependent on the nature of R(hydrogen, alkyl, alkenyl, aryl or arylene group, notably). Whilepolysiloxanes are generally linear polymers, polysilsesquioxanes have a3D structure. They self-organize in fact in different forms. Thepreparation, general properties and applications of polysilsesquioxanesare reviewed in Chem. Rev., 1995, 95(5).

Patent application JP 2001-226634 describes a coating for substrates ofdifferent natures, obtained from a polysilsesquioxane resin (havingreactive ethylenic double bonds, allowing crosslinking by radicalpolymerization) and from organic substances (copolymerizablesubstances), such as (meth)acrylic monomers. Because of the use of theseorganic substances, such a coating brought to high temperatures emitsVOCs in significant amounts.

Patent application JP 2006-047504 describes antireflective coatings,notably for LCD screens, obtained from polysilsesquioxane resins (havingboth perfluoroalkyl groups and reactive groups, such as vinyl or epoxygroups), fluorine-containing monomers, such as (meth)acrylic monomers(copolymerizable substances) and fillers, such as colloidal silica.Because of both the use of the monomers and the use of fluorinatedcompounds, such coatings brought to high temperatures emits VOCs insignificant amounts.

In such a context, the inventors desired to propose for glass orglass-ceramic substrates, a coating resistant to high temperatures,which meets the stipulations hereafter in a particularly advantageousway:

1) having satisfactory mechanical properties, notably scratchresistance,2) having satisfactory optical properties, notably opacity, colorstability (after n heating cycles),3) having good aesthetic quality,4) minimizing, or even avoiding diffusion of a product of the siliconeadhesive type across them,5) emitting an as low as possible amount of volatile organic compounds(COVs), in particular benzene, or even benzene and formaldehyde, when itis brought to a high temperature (during heating cycles),6) obtained by a method easy to carry out (notably without any lifetimeconstraint on the precursor resin), and7) able to be applied on the substrate by current techniques, such asscreen-printing.

The credit of the inventors is of having reconsidered the technicalproblem of VOC emission (most particular benzene), by polysiloxanecoatings of the prior art, when they are brought to a high temperature(emission of said VOCs in small amounts; the polysiloxane coatings ofcooking hobs developed by the Applicant have of course always been andare always compliant with the recommended thresholds), and of proposinga novel type of coating, more efficient with reference to this technicalproblem (notably in order to anticipate possible tightening ofregulations) and as efficient with reference to the other stipulationsof the specifications listed above (notably stipulations 1) and 6) ofthe specifications). Of course, the novel proposed coatingsatisfactorily adheres to the glass or glass-ceramic substrates.

According to its first object, the present invention therefore relatesto a method for preparing an article, the structure of which comprises aglass or glass-ceramic substrate and a coating fixed to at least oneportion of the surface of said substrate. It is understood, with view tothe remarks above, that said method is more particularly optimized as tothe nature of the coating, a coating capable of resisting to hightemperatures (high temperatures to which said coating is expected to besubject, because for example of its layout facing the radiant orinduction heating elements), while emitting a small amount of VOCs (inthe case in point, less VOCs (less benzene) than the coatings of theprior art with polysiloxane resin).

In a characteristic way, said preparation method comprises:

-   -   making available a substrate in a glass or a glass-ceramic;    -   applying on at least one portion of the surface of said        substrate, a layer of a dispersion:        -   the continuous phase of which essentially consists of at            least one polysilsesquioxane resin in solution in a solvent,            said at least one polysilsesquioxane resin having units of            formula (HSiO_(1.5)) and/or of formula (RSiO_(1.5)),            advantageously of formula (RSiO_(1.5)), the radicals R of            these units either being identical or not, all different            from an aryl radical, and        -   the dispersed phase of which essentially consists of mineral            fillers;    -   the heat treatment of the assembly comprising said substrate and        said layer, or of said layer mainly, so as to remove said        solvent and to cross-link said resin.

The method of the invention therefore aims at preparing an article, thestructure of which comprises a substrate in a glass or in aglass-ceramic and a coating fixed to at least one portion of the surfaceof said substrate.

The involved substrate in a glass or in a glass-ceramic is generallymore or less transparent and the involved coating is generally colored,present for purposes of decoration and/or opacification. Said coating ishowever not necessarily colored. It may notably be present for acting ondiffusion of light and giving a blurred and/or mat aspect without givingcolor.

The form of the article is conditioned by that of the substrate. Forpreparing a plate, notably a cooking hob, said substrate therefore hasthe shape of a plate (with two main substantially parallel faces andsmall thickness, said thickness is small with view to the dimensions ofsaid two main faces). Such a plate may be planar, may be essentiallyplanar with local deformations (patent application FR 2 726 550 thusdescribes plates with folded edges; patent application FR 2 777 559 thusdescribes plates with apertures with deformed perimeter), may include aplanar portion and a recessed, convex (wok type) portion, may also beformed on the whole of its surface.

In fact there are no severe limitations as to the shape of the initialsubstrate used.

In a by no means limiting way, it is indicated here that the soughtarticles advantageously consist in cooking hobs, as specified above, infireplace inserts, in fire-doors, in domestic oven doors . . . . Theshape of the initial substrate is selected for this purpose.

On at least one portion of the surface of said initial substrate,generally on at least the areas of said surface intended to withstandhigh temperatures during the use of the sought article, a dispersionlayer, precursor of a coating or of at least one constitutive layer of acoating is applied. It should however be noted that within the scope ofthe application of the method of the invention, the area for applyingthe dispersion layer, precursor of a coating or of at least oneconstitutive layer of a coating, is not necessarily localized on thesurface intended to be exposed to high temperatures, is not necessarilylocalized on the areas intended to be the hottest of said surface.Indeed, it may for example be desired to generate the coating on theouter face of oven doors (notably those of pyrolysis ovens), only at theperiphery of said outer face (in order to hide the mechanical fittingsof the door without preventing viewing of the interior of the oven).

The applied layer moreover is not necessarily of the same compositionover all its surface of application. “Different layers” for example ofdifferent colors (because of the presence of different mineral fillerswithin them or of the presence of the same fillers but at differentlevels) may be deposited on different areas of the surface ofapplication in order to give a different aspect to said areas.

The deposited layer is a dispersion. Conventionally, it contains mineralfillers (dispersed phase) in a binder (continuous phase). It ischaracterized by the original nature of said binder.

The dispersed phase of said dispersion is not original per se. Ittherefore essentially consists of mineral fillers. Said mineral fillersare likely to be involved as specified above for different purposes.They ensure at the very least a certain mechanical cohesion of thecoating. They also are advantageously involved for notably having thecoating adhere to the substrate, in order to minimize the occurrence andpropagation of cracks within said coating, in order to reinforce thesubstrate and/or to modify the aesthetic aspect of the substrate, moreparticularly for coloration purposes. Different types of fillers may beinvolved as a supplement; certain types of fillers are suitable forensuring several functions at the same time, notably those of mechanicalreinforcement and coloration. One skilled in the art knows about thesefillers, these different types of fillers and is notably aware thattheir nature and/or size and/or shape may also be optimized withreference to other parameters, such as electric conductivity.

According to advantageous alternative embodiments of the method of theinvention:

-   -   at least one fraction of the mineral fillers consists in        lamellar fillers (the light diffusion/reflection action or (and)        the mechanical reinforcement action of the coating is (are)        particularly sought);    -   at least one fraction of the mineral fillers consists in        pigments (the “coloration” action of the coating is particularly        sought).

According to a very advantageous alternative, at least one fraction ofsaid mineral fillers consists in lamellar fillers and at least onefraction of said mineral fillers consists in pigments. These may bedifferent fillers (lamellar fillers on the one hand, and pigments on theother hand) or the same fillers (lamellar pigments).

The mineral fillers may notably consist in TiO₂-coated mica platelets,in silica, alumina, borosilicates and/or oxides.

Mineral fillers are generally present in an amount from 10 to 60%,advantageously from 20 to 40%, by weight in the dispersion(dispersion=fillers+binder+solvent+optional additives). Said fillers aregenerally again found present in the heat treated layer in an amountfrom 40 to 60% by weight (for 60 to 40% by weight of heat treatedbinder).

Now, one reaches the key of the invention: the nature of the binder, thenature of the continuous phase of the dispersion, for a large partresponsible for the final properties of the coating obtained from saiddispersion, more particularly for its VOC emission, when it is broughtto a high temperature. Instead and in place of the resins (precursors)of the polysiloxane type of the prior art, the inventors recommend theuse of resins (precursors) of the polysilsesquioxane type, free of arylradicals (with reference to the problem of benzene emission).

It was not obvious to consider this type of 3D resins (crosslinked butfurther containing a few reactive groups of the —SiOH type and/orpotentially reactive groups of the SiOAlkyl type), a priori insoluble inmost usual solvents. Only the resins of this type, with low molecularweight, are soluble in the most common organic solvents. Within thescope of the method of the invention, this type of polysilsesquioxaneresin with low molecular weight is used insofar that said resin for itsapplication is in solution in a solvent. In a by no means limiting way,it may be indicated here that the involved solvent (which may strictlyconsist in a mixture of solvents) may notably be a terpenic hydrocarbon,an alcohol, such as isopropanol or terpineol, a ketone or a mixture ofthese solvents, and the resin used generally has a molecular weight (Mw)of less than 100,000 g/mol (advantageously less than 10,000 g/mol).

It was not obvious that this type of 3D aryl-free (notably phenyl-free)resin is suitable for obtaining a coating, positively meeting thedifferent stipulations recalled above. Now, such is the case.

In a characteristic way, the method of the invention therefore comprisesthe application of a layer of a dispersion, the continuous phase ofwhich essentially consists of at least one polysilsesquioxane resin notcontaining any aryl group, in solution in a solvent. The term of“essentially” excludes the presence of co-precursor organic substancesof the coating a priori capable of generating VOCs when said coating isbrought to a high temperature (it more particularly excludes thepresence of any monomer such as those disclosed in JP patentapplications: JP 2001-226634 and JP 2006-047507), but cannot exclude thepresence in small amount(s) of at least one additive, selected fromwetting agents, adhesion promoters, bubble-removing agents and otheragents. Said at least one additive, if present, is present in an amountof less than 10%, generally less than 5%, by weight, based on the (dry)resin weight. The continuous phase of the dispersion therefore containssaid at least one polysilsesquioxane resin, said solvent and optionallysaid at least one additive.

The involved polysilsesquioxane resins are with (HSiO_(1.5)) or/and(RSiO_(1.5)) units, R therefore being different from an aryl radical. Asingle polysilsesquioxane resin or a mixture of at least twopolysilsesquioxane resins (different by their formula and/or theirmolecular weight) may be present in the dispersion. Said singlepolysilsesquioxane resin or each of said polysilsesquioxane resins haveunits of either one or both of the formulae specified above, either(all) identical or not. To this day, are mainly marketedpolysilsesquioxane resins with (HSiO_(1.5)) units (so-called HSQ resinsfor Hydrogen Silsesquioxane Resin) and polysilsesquioxane resins with(RSiO_(1.5)) units, said units all containing the same radical R.However some are also found with units of different formula and in thecase in point, the synthesis of polysilsesquioxane resins having unitsof different formula does not pose any particular problem to one skilledin the art. Whatever the case, the polysilsesquioxane resin(s) used forcarrying out the method of the invention does(do) not contain any arylradical, notably no phenyl radical (with reference to the technicalproblem of benzene emission by the final coating brought to a hightemperature) and is(are) soluble in a solvent (with reference to theconstitution of the dispersion and to the application of said dispersionon at least one portion of the surface of the substrate). The presenceof any heteroatom (such as N, O, X (halogen) . . . ) in radical R isobviously implicitly excluded in reference to the technical problem ofthe emission of VOCs.

It is understood that the method of the invention provides a newcommercial outlet for polysilsesquioxane resins, as identified above.According to the present invention, a novel use for these resins asprecursors (raw materials) of (loaded) surface coatings of glass orglass-ceramic substrates is therefore proposed.

The deposited dispersion layer (the loaded original binder) generallyhas a thickness (e) between 10 and 40 μm (10 μm≦e≦40 μm). It may beapplied by any conventional technique, adapted to its viscosity. Thus,the application of this layer may notably be performed with a brush, adoctor blade, by spraying, by electrostatic deposition, by soaking, bycurtain coating, with a whirler, by the so-called inkjet technique or byscreen-printing. It is advantageously applied by screen-printing.

There is every reason to insist on the fact that with view to the natureof the resin, there is no pot life problem.

Once the substrate is covered, over at least one portion of its surface,by the dispersion, said dispersion is heat-treated. In fact, the heattreatment affects the assembly: substrate+layer (said assembly being forexample positioned in an oven) or mainly, essentially said layer(heating elements being for example positioned facing said layer andessentially heating said layer). Said heat treatment aims at removingthe solvent from the dispersion (from the deposited dispersion layer)and at crosslinking the resin of said dispersion. It was seen above thatthe involved 3D crosslinked resins further contain reactive groups(—SiOH) and/or potentially reactive groups (—SiOAlkyl) (generally atleast a few % by moles). The heat treatment crosslinks said resin viasaid reactive and/or potentially reactive groups. Said heat treatmentshould not significantly pyrolyze said resin so as to generate a coatingwith acceptable porosity and acceptable mechanical properties.

Said heat treatment is carried at a given temperature (notably in apre-heated oven) or following a thermal cycle.

Generally, said heat treatment mainly comprises the submission of theassembly: substrate+layer or of said layer mainly, to temperature(s)beyond 100° C., advantageously beyond 150° C., very advantageouslybeyond 350° C. Generally said heat treatment does not imposetemperatures beyond 550° C. (most particularly with reference to thedamageable pyrolysis phenomena mentioned above).

One skilled in the art is capable of controlling or even optimizing thetemperature/time parameters of said heat treatment.

The polysilsesquioxane resin(s) used does(do) therefore not contain anyaryl (notably benzyl) group in its(their) formula.

This may notably be at least one polysilsesquioxane resin having unitsof formula (HSiO_(1.5)) or/and (RSiO_(1.5)), the radicals R of theseunits, either identical or different, being selected from the groupconsisting in:

-   -   the linear and branched alkyl groups having 1-6 carbon atoms (of        formula: C_(n)H_(2n+1) with n=1, 2, 3, 4, 5 or 6)    -   the linear and branched alkenyl groups having 2-6 carbon atoms        (of formula: C_(n)H_(2n−1) with n=2, 3, 4, 5 or 6, when a single        ethylenic double bond is present; the presence of more than one        ethylenic double bond being not at all excluded); and    -   the cycloalkyl groups having 3-6 carbon atoms (of formula        C_(n)H_(2n−1) with n=3, 4, 5 or 6).

Advantageously, said at least one polysilsesquioxane resin has(RSiO_(1.5)) units with R (either identical or different) selected fromthe group consisting in the (linear and branched) C₁-C₆ alkyl groups andthe C₃-C₆ cycloalkyl groups. Such resins generate coatings which emit noor only very little benzene when they are brought to a high temperature.

Very advantageously, said at least one polysilsesquioxane resin has(RSiO_(1.5)) units, the radicals R of the different units (eitheridentical or different) being selected from the group consisting in thelinear and branched alkyl groups including 1-3 carbon atoms and thecycloalkyl groups including 3-6 carbon atoms. The corresponding coating,brought to a high temperature, emits no or very little benzene and haslimited emission of formaldehyde.

According to an alternative, the radicals R of all the units of theresin(s) used (with (RSiO_(1.5)) units) are identical. They areadvantageously selected from the groups identified above. According toanother alternative, the resin used is an HSQ resin (with (HSiO_(1.5))units).

According to a particularly preferred alternative, thepolysilsesquioxane resin used is with identical units (RSiO_(1.5)), withR=a methyl group (CH₃). Such resins emit no or very little benzene andemit less formaldehyde than polysiloxane resins of the prior art. Theiruse within the scope of the application of the method of the inventionis particularly recommended.

The layer of the dispersion with original binder may be deposited on thesurface (on at least one portion of said surface) of the exposed (bare)substrate. It may also be deposited on at least one layer depositedbeforehand on said surface.

This at least one sub-layer is opportunely present with reference to thecoloration and/or scratch resistance of the final coating. This at leastone sub-layer should not obviously be capable of emitting benzene oranother VOC in significant amounts. The idea is not ruin the positiveeffects of the use of polysilsesquioxane resin (free of any arylradical). Said at least one sub-layer is opportunely per se a minerallayer or a precursor layer of a mineral layer or of an essentiallymineral layer. This may notably be an enamel, a metal coating giving amirror aspect, a coating with a loaded sol-gel type binder (this lasttype of coating by undergoing heat treatment is totally or partlymineralized). The presence of an organic portion in the final sub-layermay actually be tolerated, insofar that it remains minimal (it may thusremain minimal because of the nature of the precursor sub-layer (with alow organic fraction) and/or because of the heat treatment and/orbecause of the small thickness of the final layer).

This at least one sub-layer is deposited by methods known per se, forexample by sputtering, by physical vapor phase deposition, byscreen-printing . . . .

The method of the invention may strictly only comprise the applicationof a single dispersion layer with an original binder on the exposed(bare) substrate or on the substrate already coated with at least onesub-layer as specified above.

According to other alternative embodiments, it further comprises aftersaid application of said dispersion layer with an original binder,before and/or after said heat treatment, the application of at leastanother layer (of course not capable of emitting VOCs in significantamounts because of its nature and/or its localization, this in order notto ruin the positive effects of the use of the polysilsesquioxaneresin), generally the application:

-   -   either (generally in a discontinuous way), on only at least one        area not intended to be subject to high temperatures (a        so-called cold area), of an additional organic layer containing        mineral fillers, precursor of a layer capable, when brought to        high temperature, of emitting benzene (significant amounts of        benzene);    -   or, on whole or part of the surface of said substrate, of an        additional layer of a dispersion, the continuous phase of which        essentially consists of at least one polysilsesquioxane resin in        solution in a solvent, said at least one polysilsesquioxane        resin having units of formula (HSiO_(1.5)) or/and of formula        (RSiO_(1.5)), advantageously of formula (RSiO_(1.5)), the        radicals R of these units either being identical or not, all        different from an aryl radical, and the dispersed phase of which        essentially consists of mineral fillers.

Within the scope of the first alternative, it is possible to afforddepositing precursor layers of resins capable of emitting benzene (andother VOCs) when they are brought to a high temperature (for example,precursor layers of resins used according to the prior art, of thepolysiloxane type with phenyl groups), insofar that said resins are notcapable of being brought to a high temperature, insofar that they areonly deposited on cold area(s). Within the scope of this firstalternative, the deposited additional organic layer (containing mineralfillers) regardless of its exact nature, is a precursor of a layerwithout any particular restriction as to the emission of benzene when itis brought to a high temperature.

Within the scope of the second alternative, a dispersion layer of atleast one polysilsesquioxane resin not capable of releasing benzene (insignificant amounts) is deposited. This layer may be deposited both inhot area(s) and in cold area(s). It intervenes on all or part of therelevant surface. This is therefore a second layer of thepolysilsesquioxane resin type (free of any aryl radical). Said secondlayer is not necessarily identical with the first.

The layers deposited within the frame of said first and secondalternatives are also layers containing mineral fillers. It is seen thatthe mechanical cohesion of the resulting coating is at stake. Such alayer advantageously intervenes in order to increase the scratchresistance of the final coating. This may notably be a protective layeron a decorative layer.

The method of the invention may therefore be carried out according tomany alternatives and includes:

-   -   the optional application of at least one sub-layer,    -   the application of a dispersion layer, the continuous phase of        which essentially consists of a polysilsesquioxane resin without        any aryl group, and    -   the optional application of an additional layer of one of the        two types specified above (a generally discontinuous layer, (of        the polysiloxane type, for example) according to the prior art        or a new continuous or discontinuous layer of the        polysilsesquioxane type, recommended according to the invention        as a base layer). Both polysilsesquioxane resins of both layers        are either of identical nature or not. Advantageously, they are        of same nature (either loaded or not in an identical way).

It was seen above that the method of the invention is advantageouslyapplied for preparing an article such as a cooking hob.

According to an advantageous alternative, the method of the inventioncomprises:

-   -   making available a plate in a glass-ceramic or in a glass, and    -   applying a layer of the dispersion (with a polysilsesquioxane        binder without any aryl radical), on all or part of the surface        of both (main) faces of said plate or on all or part of the        surface of a single (main) face of said plate.

Within the scope of this advantageous alternative, the method of theinvention may comprise an application by screen-printing, over the wholesurface of one face of said plate, of a dispersion layer, the continuousphase of which essentially consists of a polysilsesquioxane resin withunits of formula (CH₃SiO_(1.5)) in solution in a solvent and thedispersed phase of which essentially consists of lamellar pigments;followed by a heat treatment of the assembly comprising said plate andsaid layer at temperature(s) above 350° C. and by an application byscreen-printing of an additional layer on at least the cold areas (areaswhich are not intended to be subject to high temperatures) of said faceof said plate.

Said application by screen-printing over the whole surface of one faceof said plate (the one intended to be positioned facing the heatingelements) is carried out, as indicated above, on said exposed surface orcovered beforehand with a mineral coating.

According to its second object, the present invention relates toarticles able to be obtained after carrying out the method of theinvention as described above. Said articles have a structure whichcomprises a substrate in a glass or in a glass-ceramic and a coatingfixed to at least one portion of the surface of said substrate.

In a characteristic way, said coating was obtained by the method of theinvention: from at least one resin of the polysilsesquioxane type resinwithout any aryl radical (notably without any phenyl radical) containingmineral fillers.

Considering the above statements, it is understood that said coating,either continuous or not, is able of comprising several layers of samenature or of different nature (for example: an optional mineral oressentially mineral sub-layer+a layer derived from at least one loadedpolysilsesquioxane resin without any aryl radical+an optional overlayerderived from a resin of the prior art, for example of the loadedpolysiloxane type or of at least one loaded polysilsesquioxane resinwithout any aryl radical).

The articles of the invention advantageously consist in cooking hobs,(intended to be) arranged out above heating elements, with their faceincluding said coating or one of their faces including said coatingpositioned facing said heating elements. Said heating elementsadvantageously consist in induction heating elements.

The present invention is now illustrated in a by no means limiting way,by the examples (comparative examples and inventive examples) hereafter.Its advantage emerges from considering in parallel said examples andcomparative examples. It is most particularly illustrated in theappended FIG. 1.

Said FIG. 1 shows the relative variation (%) of viscosity (Brookfieldviscosity (mPa·s)) versus time (days), of a dispersion, precursor of acoating of the prior art (see comparative Example 1 hereafter) and of adispersion, precursor of a coating of the invention (see Example 1hereafter). The conditions for measuring the viscosity are specifiedhereafter (point I.2)).

I. COMPARATIVE EXAMPLES 1 AND 2 AND EXAMPLES 1 TO 3 OF THE INVENTION 1]Raw Materials

The main raw materials identified hereafter were used.

Solvent: terpineol (CAS 8000-41-7) marketed by Sigma.

Mineral fillers: different types of fillers were used:

-   -   pigments marketed by MERCK under the commercial names of Iriodin        123 (white pigment) and Iriodin 323 (gold pigment): these are        mica platelets coated with TiO₂;    -   other ones marketed by Ciba under the commercial name of Xymara        Satin Black Pearl (black pigment): these are also coated mica        platelets;    -   other ones marketed by Ferro under the commercial name Ferro        240137 (black pigment): these are cobalt, chromium, iron and        nickel oxides.

Binder:

-   -   for the comparative Example 1, a sol-gel type resin (noted as        CS1) was prepared from methyltriethoxysilane (MTEOS) and from        tetraethylorthosilicate (TEOS): see hereafter;    -   for the Examples 1-3 of the invention, a commercial powder of        polymethylsilsesquioxane (PMS) resin (marketed by Wacker (DE)        under the commercial name of Belsil PMS-MK) was used in a        characteristic way.

The resin has the chemical formula (CH₃SiO_(1.5))_(n) with n=130-150; ithas a density of 1.1 g/cm³ and its average molecular weight (Mw) is9,400 g/mol. It contains 4% by moles of ethoxy functions.

2] Preparation of the Dispersion

a) As regards the comparative Example 1, the raw materials identified inTable 1 hereafter were used in the amounts indicated in said Table 1.

The silicone base CS1 (a base of the sol-gel type (MTEOS/TEOS), of thetype of the one of Example 3 of U.S. Pat. No. 6,836,923) was prepared asfollows. 250 g of methyltriethoxysilane [CAS 2031-67-6] and 87 g oftetraethylorthosilicate [CAS 78-10-4] were loaded into a 1 liter flask.141 g of colloidal silica solution Ludox HS30 [CAS 7631-86-9]were addedto this mixture under vigorous stirring. The mixture still understirring was then cooled to 0° C. and 2.53 g of 37% by weighthydrochloric acid were added dropwise. After 30 minutes, the reactionmedium was diluted by adding 300 g of terpineol [CAS 8000-41-7]. Themixture was then left under stirring at room temperature for 16 hours.Finally, about 269 g of ethanol produced by the hydrolysis reaction,were removed under reduced pressure with a rotary evaporator.

To the thereby obtained silicone base CS1, were added pigments, oneadditive (silicone oil (noted as PDMS)) involved with reference towettability and coating and butanol as a co-solvent with which theviscosity may be adjusted, as indicated in Table 1.

The resulting composition (dispersion) was mixed using a three-cylindermixer.

b) As regards Examples 1-3, the polymethylsilsesquioxane resin wasdissolved in an amount of 50% by weight, into the terpineol, understirring, at room temperature, in order to obtain the silicone base MS1.

To said silicone base MS1, were added the pigments, an additive(silicone oil (noted as PDMS)) involved with reference to wettabilityand to coating, and butanol as a co-solvent with which the viscosity maybe adjusted, as indicated in said Table 1.

The resulting compositions (dispersions) were mixed using athree-cylinder mixer.

TABLE 1 Comparative Compositio Example 1 Example 1 Example 2 Example 3Silicone CS1 37.5 g MS1 37.5 g MS1 37.5 g MS1 37.1 g  base PigmentIriodin 19.5 g Iriodin 19.5 g Iriodin   21 g Xymara  15 g 123 123 323Satin Black Pearl Pigment Ferro  1.5 g Ferro  1.5 g 240137 240137Additive PDMS  1.5 g PDMS  1.5 g PDMS  1.5 g PDMS 1.5 g Co-solventButanol   3 g Butanol   3 g Butanol   1 g Butanol 4.5 g

The dispersions of comparative Example 1 and of Example 1 are pale greycompositions (the grey is obtained by mixing the white pigment Iriodin123 with the black pigment Ferro 240137). The dispersion of Example 2has a gold color, the one of Example 3 is dark grey. Different coatingcolors may thereby be obtained easily by carrying out the method of theinvention (with different pigments).

The dispersions of comparative Example 1 and Example 1 are thereforedifferent by the nature of their binder. They differ further by theirpreparation method. The simplicity of the method for preparing thedispersion according to Example 1 very clearly shows the actualadvantage of the method of the invention (within the scope of itsapplication, there is no constraint with reference to the lifetime ofthe dispersion (insofar that said dispersion is stable, which is not thecase of the dispersion according to the prior art (see hereafter as wellas appended FIG. 1)).

Monitoring the time stability (lifetime) of the dispersions ofcomparative Example 1 and of Example 1 was achieved by measuring theviscosity of said dispersions. For this purpose, a Brookfield model DV-IPrime viscosimeter was used. The measurements were conducted with moduleNo. 6 and a speed of rotation of 100 rpm. The results of saidmeasurements are indicated in Table 2 hereafter. They are also marked onthe appended FIG. 1.

TABLE 2 Comparative Example 1 Example 1 Time Viscosity RelativeViscosity Relative (day) (mPa · s) variation (mPa · s) variation 0 37600.0% 3120 0.0% 7 4180 11.2% 2850 −8.7% 14 4570 21.5% 3000 −3.8% 21 508035.1% 3120 0.0%

While a change in viscosity is clearly identifiable for the dispersionof comparative Example 1 (a 35% increase after 3 weeks), the viscosityof the dispersion of Example 1 remains stable, which gives a clearadvantage to the method of the invention.

3] Deposition of Dispersions (Compositions) on a Glass-Ceramic Plate

The deposits were applied by screen-printing, by using screens of 32wires/cm² on one face of transparent glass-ceramic plates of theKeralite® type (marketed by the Applicant).

The deposited layers have a thickness of about 25 μm.

4] Heat Treatment

The coated glass-ceramic plates are dried for 30 min at 120° C.

Next, they undergo the hereafter thermal cycle in a static oven:

-   -   rise from 25° C. to 350° C. in 5 min    -   maintaining 350° C. for 20 min    -   rise from 350° to 480° C. in 20 min    -   maintaining 480° C. for 5 min    -   fast cooling to 25° C.

5] Properties of the Thereby Obtained Coatings

The main properties of the obtained different pale grey coatings weremeasured as follows.

-   -   Scratch resistance was evaluated by visual examination of the        upper side of the glass-ceramic plate (on the side opposite to        the grey layer). Several scratch lines are formed, at different        forces, with the scratch Hardness Tester TQC SP0010 on the        layer. The value of scratch resistance expressed in Newtons (N),        is then the value for which no marking may be observed on the        upper side of the glass-ceramic with the highest force value.    -   Benzene emission was measured, by a comparative method, by        direct thermal desorption at 400° C. for 1 hr coupled with gas        chromatography. The amounts of emitted benzene are calculated by        numerical integration of the benzene peak obtained on the        chromatogram, based on the analyzed amount of sample. The        results are then normalized with, as unit value, the result        obtained for the comparative Example 1.    -   Formaldehyde emission was measured after 2 hrs at 400° C., by        trapping it in a functionalized column with        dinitrophenylhydrazine, by extracting the formed hydrazone and        by analysis with high pressure liquid chromatography (HPLC).    -   The thickness was measured with a surface roughness profilometer        Hommel Tester T2000.    -   The opacity test was positive if an object placed behind the        coating was not visible.    -   The aesthetic quality was evaluated by visual examination.    -   The stability of the color was evaluated (relatively to a        reference) after maintaining the coated plate at 575° C. for 10        min.

The obtained results of the measurements appear in Table 3 below.

One is also interested in the same way in the properties of coatings ofplates of the prior art. For comparative Example 2, the pale greydecorative coating of glass-ceramic plates available at the Applicant(Eurokera) under the Keraresin® brand was considered; said coating isbased on a methylphenylsiloxane polymer.

TABLE 3 Comparative Comparative Example 1 Example 2 Example 1 Color GreyGrey Grey Scratch resistance, R (Newtons) R < 1 1 < R < 2 1 < R < 2Benzene emission 1 93 0.19 Formaldehyde emission (mg/m²) 7.2 12 8Thickness (μm) 30 25 27 Opacity − + + Aesthetic quality OK OK OK Colorstability + +

This table shows that with the method of the invention carried out withthe composition of Example 1 (polysilsesquioxane resin without any arylradical) a decorative coating may be obtained, which meets thespecifications listed in the introduction of the present text. Saidcoating has satisfactory scratch resistance (sufficient for its use),good opacity, good aesthetic quality and good color stability. It emitsvery small amounts of benzene and less formaldehyde than the coating ofcomparative Example 2.

The coating of comparative Example I (obtained from a sol-gel resin) hasvery poor mechanical properties, which confirms the importance of thenature of the binder. Further, its opacity is slightly not as good asthat of the coatings of comparative Example 2 and Example 1.

The coating of comparative Example 2 (coating of the prior art)expectedly has suitable mechanical and aesthetic properties. However,under the test conditions, it releases more benzene and formaldehydethan the coating according to the invention.

II. EXAMPLES 4 AND 5 OF THE INVENTION

The scratch resistance of the decorative coatings of the plates may beimproved by depositing a protective layer on the decorative layer(according to the invention).

Plates coated according to Example 1 above (grey coating of theinvention) were then covered with a second layer, the composition ofwhich is specified in Table 4 below. This composition in acharacteristic way contains the silicone base MS1 (see above: based onthe polymethylsilsesquioxane resin).

TABLE 4 Composition Example 4 Example 5 Silicone base MS1 37.5 g MS137.5 g Pigment Iriodin 123   21 g Xymara Satin   18 g Black PearlPigment Xymara Satin Black  2.5 g Pearl Co-solvent Butanol   6 g Butanol 7.5 g

The second layer is deposited by screen-printing under the sameconditions (as the first layer).

After depositing said second layer, the plates are treated at 400° C.for 30 min, in the static oven. This heat treatment under less severeconditions (than those of the heat treatment of the first layer),enables a protective layer to be obtained which has better mechanicalproperties. The properties of glass-ceramics coated with both coatingswere measured under the same conditions as earlier. The obtained resultsappear in Table 5 hereafter.

In the first part of said Table 5, the results obtained with the singlecoating (of the prior art) of comparative Example 2 above and the singlecoating (of the invention) of Example 1 above are reported.

TABLE 5 Comparative Example 2 Example 1 Example 4 Example 5 Color of thedecorative Grey Grey Grey Grey coating Scratch resistance, R R < 1 < 2 1< R < 2 4 9 (Newtons) Benzene emission 93 0.19 0.27 0.27 Formaldehyde 128 14 10 emission (mg/m²) Thickness (μm) 25 27 51 48 Opacity + + + +Aesthetic quality + + + + Color stability + + + +

The bilayer coatings of Examples 4 and 5 have improved scratchresistance. They release very small amounts of benzene as compared withthe coating of comparative Example 2 and very little formaldehyde.

1. A method for preparing an article, the structure of which comprises aglass or glass-ceramic substrate and a coating fixed to at least oneportion of a surface of said substrate, said method comprising: makingavailable a substrate of a glass or a glass-ceramic; applying on atleast one portion of a surface of said substrate, a layer of adispersion: a continuous phase of which essentially consists of at leastone polysilsesquioxane resin in solution in a solvent, said at least onepolysilsesquioxane resin having units of formula (HSiO_(1.5)) and/or offormula (RSiO_(1.5)), the radicals R of these units either beingidentical or not, and all different from an aryl radical, and adispersed phase consisting essentially of mineral fillers; heat-treatingthe layer-coated substrate or the layer so as to remove said solvent andto crosslink said resin.
 2. The method according to claim 1, whereinsaid at least one polysilsesquioxane resin has units (RSiO_(1.5)), theradicals R of said units, either identical or not, being selected fromthe group consisting of linear and branched alkyl groups having 1 to 6carbon atoms and the cycloalkyl groups having 3 to 6 carbon atoms. 3.The method according to claim 1, wherein said at least onepolysilsesquioxane resin has units (RSiO_(1.5)), the radicals R of saidunits, either identical or not, being selected from the group consistingof linear and branched alkyl groups having 1 to 3 carbon atoms and thecycloalkyl groups having 3 to 6 carbon atoms.
 4. The method according toclaim 1, wherein said at least one polysilsesquioxane resin is apolysilsesquioxane resin with units (CH₃SiO_(1.5)).
 5. The methodaccording to claim 1, wherein at least one fraction of said fillersconsists of lamellar fillers.
 6. The method according to claim 1,wherein at least one fraction of said fillers consists of pigments. 7.The method according to claim 1, wherein said application of said layeris performed with a brush, with a doctor blade, by spraying, byelectrostatic deposition, by soaking, by curtain coating, with awhirler, by ink jet printing, or by screen-printing.
 8. The methodaccording to claim 1, wherein said heat treatment comprises subjectingsaid assembly or said layer mainly to a temperature above 150° C.
 9. Themethod according to claim 1, wherein said application of said layer isperformed on at least one layer deposited beforehand; said at least onelayer being a mineral layer or a layer precursor of a mineral layer orof an essentially mineral layer.
 10. The method according to claim 1,further comprising, after said application of said layer, and beforeand/or after said heat treatment, applying: on only at least one areanot intended to be subject to high temperatures, an additional organiclayer containing mineral fillers, precursor of a layer capable, whenbrought to high temperatures, of emitting benzene; or on all or part ofthe surface of said substrate, an additional layer of a dispersion, thecontinuous phase of which essentially consists of at least onepolysilsesquioxane resin in solution in a solvent, said at least onepolysilsesquioxane resin having units of formula (HSiO_(1.5)) and/or offormula (RSiO_(1.5)), the radicals R of these units either beingidentical or not, and all different from an aryl radical and thedispersed phase of which essentially consists of mineral fillers. 11.The method according to claim 1, wherein said substrate is a plate andsaid application of said layer is performed on all or part of thesurface of both faces of said plate or on all or part of the surface ofa single face of said plate.
 12. The method according to claim 11,comprising applying by screen-printing on the whole surface of a face ofsaid plate, a layer of a dispersion, the continuous phase of whichessentially consists of a polysilsesquioxane resin with units of formula(CH₃SiO_(1.5)) in solution in a solvent and the dispersed phase of whichessentially consists of lamellar pigments; followed by heat treatment ofthe assembly comprising said plate and said layer at a temperature above350° C. and by the application by screen-printing of an additional layeron at least unheated areas of said face of said plate.
 13. An article,the structure of which comprises a substrate of a glass or of aglass-ceramic and a coating fixed to at least one portion of a surfaceof said substrate, able to be obtained after carrying out the methodaccording to claim
 1. 14. The article according to claim 13, comprisinga cooking top adapted to be laid out above heating elements, with itsface including said coating or one of its faces including said coatingpositioned facing said heating elements.